440 research outputs found
Sagnac effect in a chain of mesoscopic quantum rings
The ability to interferometrically detect inertial rotations via the Sagnac
effect has been a strong stimulus for the development of atom interferometry
because of the potential 10^{10} enhancement of the rotational phase shift in
comparison to optical Sagnac gyroscopes. Here we analyze ballistic transport of
matter waves in a one dimensional chain of N coherently coupled quantum rings
in the presence of a rotation of angular frequency, \Omega. We show that the
transmission probability, T, exhibits zero transmission stop gaps as a function
of the rotation rate interspersed with regions of rapidly oscillating finite
transmission. With increasing N, the transition from zero transmission to the
oscillatory regime becomes an increasingly sharp function of \Omega with a
slope \partialT/\partial \Omega N^2. The steepness of this slope dramatically
enhances the response to rotations in comparison to conventional single ring
interferometers such as the Mach-Zehnder and leads to a phase sensitivity well
below the standard quantum limit
Molecule formation as a diagnostic tool for second order correlations of ultra-cold gases
We calculate the momentum distribution and the second-order correlation
function in momentum space, for molecular dimers
that are coherently formed from an ultracold atomic gas by photoassociation or
a Feshbach resonance. We investigate using perturbation theory how the quantum
statistics of the molecules depend on the initial state of the atoms by
considering three different initial states: a Bose-Einstein condensate (BEC), a
normal Fermi gas of ultra-cold atoms, and a BCS-type superfluid Fermi gas. The
cases of strong and weak coupling to the molecular field are discussed. It is
found that BEC and BCS states give rise to an essentially coherent molecular
field with a momentum distribution determined by the zero-point motion in the
confining potential. On the other hand, a normal Fermi gas and the unpaired
atoms in the BCS state give rise to a molecular field with a broad momentum
distribution and thermal number statistics. It is shown that the first-order
correlations of the molecules can be used to measure second-order correlations
of the initial atomic state.Comment: revtex, 15 pages,8 figure
Phase Conjugation of a Quantum-Degenerate Atomic Fermi Beam
We discuss the possibility of phase-conjugation of an atomic Fermi field via
nonlinear wave mixing in an ultracold gas. It is shown that for a beam of
fermions incident on an atomic phase-conjugate mirror, a time reversed backward
propagating fermionic beam is generated similar to the case in nonlinear
optics. By adopting an operational definition of the phase, we show that it is
possible to infer the presence of the phase-conjugate field by the loss of the
interference pattern in an atomic interferometer
Spin current and shot noise from a quantum dot coupled to a quantized cavity field
We examine the spin current and the associated shot noise generated in a
quantum dot connected to normal leads with zero bias voltage across the dot.
The spin current is generated by spin flip transitions induced by a quantized
electromagnetic field inside a cavity with one of the Zeeman states lying below
the Fermi level of the leads and the other above. In the limit of strong
Coulomb blockade, this model is analogous to the Jaynes-Cummings model in
quantum optics. We also calculate the photon current and photon current shot
noise resulting from photons leaking out of the cavity. We show that the photon
current is equal to the spin current and that the spin current can be
significantly larger than for the case of a classical driving field as a result
of cavity losses. In addition to this, the frequency dependent spin (photon)
current shot noise show dips (peaks) that are a result of the discrete nature
of photons
Measuring dark energy spatial inhomogeneity with supernova data
The gravitational lensing distortion of distant sources by the large-scale
distribution of matter in the Universe has been extensively studied. In
contrast, very little is known about the effects due to the large-scale
distribution of dark energy. We discuss the use of Type Ia supernovae as probes
of the spatial inhomogeneity and anisotropy of dark energy. We show that a
shallow, almost all-sky survey can limit rms dark energy fluctuations at the
horizon scale down to a fractional energy density of ~10^-4Comment: 4 pages; PRL submitte
A two measure model of dark energy and dark matter
In this work we construct a unified model of dark energy and dark matter.
This is done with the following three elements: a gravitating scalar field, phi
with a non-conventional kinetic term, as in the string theory tachyon; an
arbitrary potential, V(phi); two measures -- a metric measure (sqrt{-g}) and a
non-metric measure (Phi). The model has two interesting features: (i) For
potentials which are unstable and would give rise to tachyonic scalar field,
this model can stabilize the scalar field. (ii) The form of the dark energy and
dark matter that results from this model is fairly insensitive to the exact
form of the scalar field potential.Comment: 8 pages,no figures, revtex, typos corrected to match published
versio
Boson-Fermion coherence in a spherically symmetric harmonic trap
We consider the photoassociation of a low-density gas of quantum-degenerate
trapped fermionic atoms into bosonic molecules in a spherically symmetric
harmonic potential. For a dilute system and the photoassociation coupling
energy small compared to the level separation of the trap, only those fermions
in the single shell with Fermi energy are coupled to the bosonic molecular
field. Introducing a collective pseudo-spin operator formalism we show that
this system can then be mapped onto the Tavis-Cummings Hamiltonian of quantum
optics, with an additional pairing interaction. By exact diagonalization of the
Hamiltonian, we examine the ground state and low excitations of the Bose-Fermi
system, and study the dynamics of the coherent coupling between atoms and
molecules. In a semiclassical description of the system, the pairing
interaction between fermions is shown to result in a self-trapping transition
in the photoassociation, with a sudden suppression of the coherent oscillations
between atoms and molecules. We also show that the full quantum dynamics of the
system is dominated by quantum fluctuations in the vicinity of the
self-trapping solution.Comment: 16 pages, 14 figure
Quantum bistability and spin current shot noise of a single quantum dot coupled to an optical microcavity
Here we explore spin dependent quantum transport through a single quantum dot
coupled to an optical microcavity. The spin current is generated by electron
tunneling between a single doped reservoir and the dot combined with intradot
spin flip transitions induced by a quantized cavity mode. In the limit of
strong Coulomb blockade, this model is analogous to the Jaynes-Cummings model
in quantum optics and generates a pure spin current in the absence of any
charge current. Earlier research has shown that in the classical limit where a
large number of such dots interact with the cavity field, the spin current
exhibits bistability as a function of the laser amplitude that drives the
cavity. We show that in the limit of a single quantum dot this bistability
continues to be present in the intracavity photon statistics. Signatures of the
bistable photon statistics manifest themselves in the frequency dependent shot
noise of the spin current despite the fact that the quantum mechanical average
spin current no longer exhibits bistability. Besides having significance for
future quantum dot based optoelectronic devices, our results shed light on the
relation between bistability, which is traditionally viewed as a classical
effect, and quantum mechanics
Cosmological Evolution of a Tachyon-Quintom Model of Dark Energy
In this work we study the cosmological evolution of a dark energy model with
two scalar fields, i.e. the tachyon and the phantom tachyon. This model enables
the equation of state to change from to in the evolution of
the universe. The phase-space analysis for such a system with inverse square
potentials shows that there exists a unique stable critical point, which has
power-law solutions. In this paper, we also study another form of
tachyon-quintom model with two fields, which voluntarily involves the
interactions between both fields.Comment: 17 pages, 10 figure
Discriminating Electroweak-ino Parameter Ordering at the LHC and Its Impact on LFV Studies
Current limit on the dark matter relic abundance may suggest that
should be smaller than prediction in the minimal supergravity scenario (mSUGRA)
for moderate and . The electroweak-ino parameter and
are then much closer to each other. This can be realized naturally in
the non-universal Higgs mass model (NUHM). Since the heaviest neutralino
() and chargino () have significant gaugino
components, they may appear frequently in the left-handed squark decay and then
be detectable at the LHC. In such a case, we showed that the hierarchy of and can be determined. In the light slepton mass scenario with
non-vanishing lepton-flavor violation (LFV) in the right-handed sector, NUHM
with small corresponds to region of parameter space where strong
cancellation among leading contributions to can occur. We
showed that determination of electroweak-ino hierarchy plays a crucial role in
resolving cancellation point of and determination of LFV
parameters. We also discussed test of the universality of the slepton masses at
the LHC and the implications to SUSY flavor models.Comment: 34 pages, 16 figure
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